Many micro-electromechanical systems (MEMS) require an encapsulation under vacuum or under a controlled atmosphere and pressure to ensure either a good performance or an acceptable lifetime of operation. The encapsulation has to be performed without the deposition of sealing material on the MEMS device, which can cause damage to the device. In “The indent reflow sealing (IRS) technique—a method for fabrication of sealed cavities for MEMS devices”, J. MEMS, 9, p. 206-217, 2000 by H. A. T. Tilmans, M. D. J. van de Peer and E. Beyne, which is incorporated by reference herein in its entirety, two approaches for wafer-scale zero-level packaging are mentioned.
The most popular approach is based on wafer bonding. Here, the sealing is performed by connecting two wafers (device wafer and capping wafer) together by means of a reflowable material. An example of such packaging is described in patent EP-A-0951069, which is incorporated by reference herein in its entirety. In this approach an expensive substrate, e.g. a Si wafer or a MEMS substrate, is used as a cap to close the cavity comprising the MEMS device. These wafers are thick and the sealing ring is large. Therefore, the resulting encapsulated MEMS device is space consuming. Moreover, batch processing is not possible.
Alternatively, encapsulation can be done by the fabrication and sealing of surface micro-machined membranes. The use of conformal LPCVD (low-pressure chemical vapour deposition) films is a known method for encapsulation at low pressure [C. Liu, Y. C. Tai, “Sealing of micro machined cavities using chemical vapour deposition methods: characterisation and optimisation”, J. MEMS, 8, p. 135-145, 1999, which is incorporated by reference herein in its entirety]. The sealing of the cavity comprising the MEMS devices is done while depositing the conformal film. Hence, the atmosphere and pressure of the sealed MEMS device are those of the deposition chamber. Methods for sealing at higher pressures up to the order of atmospheric pressure and a few times that value, by the deposition of thin films, are however not widespread. Moreover, most of these atmospheric pressure techniques do not prevent material deposition inside the cavity.
MEMS devices can be very fragile and deposition of material on the device is preferably avoided. As disclosed in “Silicon processing for the VLSI era, vol. 1—Process Technology”, 2nd ed., 2000, S. Wolf and R. N. Tauber, chapter 11.8, pages 475-7, which is incorporated by reference herein in its entirety, hole filling may be performed by sputtering, e.g. in the formation of a via. This technique is not suitable for sealing cavities in which fragile devices are located, i.e. in cases in which the fragile device would be buried in the filling substance.
In EP0783108 A1, which is incorporated by reference herein in its entirety, a method is disclosed for the closure of openings in a membrane layer of, for instance, polycrystalline silicon which is covering a cavity. Hereby a layer of doped glass is deposited on the membrane layer, such that holes of certain dimensions are not closed but slightly narrowed. A temperature step is then applied, whereby the doped glass material further enters the cavity, creeping along the lower or inner surface of the film and later along the cavity walls, such that a layer of glass is formed covering the walls of the cavity. If the openings are small enough, the underlying cavity can be sealed by this temperature step, whereby the openings are completely filled with glass material.
In U.S. Pat. No. 6,337,499 B1, which is incorporated by reference herein in its entirety, trenches or holes in a substrate, called cavities, are sealed by depositing a layer of doped glass on top of the substrate, hereby narrowing the opening of the cavity. A reflow step is then performed whereby the cavity is sealed by the reflowed glass layer, which partially enters the cavity. Optionally, deposition of a passivation layer, having an substantially constant thickness of for instance 50 nm, is performed. It is considered desirable for the remaining cavity to have a constant width of about 1 μm over a depth T of typical 40 μm.